Did you just have the muffler installed and left the rest of the pipe intact? If you swap out to a larger diameter it should run more than that...otherwise you got one heck of a deal smile Mine was for 3" pipe at the Y all the way out which is why mine was so pricey (something about that diameter really jacks up the price of a system...).
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Charlie

Sensitivity is important in any relationship...
"I don't like country music, but I don't mean to denigrate those who do. And for the people who like country music, denigrate means 'put down.'" --Bob Newhart